Railway vehicles in most countries show a similar basic design which, inter alia, includes that both ends of the vehicles are provided with a coupling device in the middle, surrounded by two buffers. The main task of the coupling device is to transfer traction forces from one vehicle to another, whilst the buffers have to take care of compressive forces or impacts between the vehicles elastically in order to reduce impact loads on structure or cargo to a harmless level.
The heaviest demands upon the function of buffers come from shunt yard operation, when railway wagons often bump together at considerable speeds. The resistance to closure of the buffers, combined with their stroke length, must be sufficient to absorb the energy of normally occuring impacts, because if a buffer reaches the end of its stroke, the system becomes rigid, and the remaining impact energy might lead to damage.
It can be mentioned that a coupling system different from the one described above, is used by several railways around the world, e.g. at the iron ore railroad between Lulea and Narvik in Northern Scandinavia. In this case, the vehicles are provided with a central draw gear in which a built-in shock absorber has the same task as the separate buffers mentioned above. The demands upon such draw gears or central couplings regarding shock absorption are, of course, the same as when separate buffers are used, and the term "buffer" in the following description is to be understood as "draw gear shock absorber" when applicable.
Due to practical reasons, it is not possible to design buffers with an extremely long stroke, and a considerable energy absorption capacity thus implies a considerable resistance to closure. If buffers are designed to take care of impacts between heavy wagons at quite high speeds, it is difficult to avoid that such buffers cause a brutal deceleration to a light wagon running into a heavy one at a moderate speed. The heavy wagon is in this case almost immovable, and the deceleration of the light one is equal to the buffer force divided by the mass of the wagon.